An oxygen gas in-situ measurement method for infrared laser spectrum and a device thereof belong to the gas detection technical field. The device comprises a main case, an in-situ measurement module and a shielded transmission cable, wherein a power switch, a data transmission interface, a liquid crystal display screen, a power socket, a fan and a first cable interface are mounted on the main case; a data acquisition processing and controlling module, a direct current voltage-stabilized module, a signal filter and amplify circuit, a temperature control module, a current control module, a lock-phase amplifier and a modulation signal generator are arranged in the main case; the in-situ measurement module comprises a second cable interface, a calibration pool slot, a gas outlet fast plug interface, a gas inlet gas plug interface, a first spherical mirror, a second spherical mirror, a semiconductor laser, a quartz window flake, a tungsten steel support rod, an auxiliary lens base, an auxiliary lens, a main lens, a main lens base, a sealing cover and a photoelectric detector. The measurement method combines semiconductor laser adsorption spectrum, wavelength modulation and harmonic wave detection and open-type long optical path reflection adsorption pool. The invention has sensitive detection.